(1) The sera of undifferentiated nasopharyngeal carcinoma (NPC) patients in active disease but not in remission contained a lymphocyte stimulation inhibitor (LSI) which abrogated the response of sensitized lymphocytes from Epstein-Barr virus (EBV) seropositive individuals to EBV antigens. LSI was not detected in sera from EBV-seropositive or negative healthy individuals, nor in sera from patients with non-NPC carcinomas of the head and neck, and thus may prove an NPC marker of clinical, diagnostic and prognostic significance. (2) The role of EBV in immunodeficiency diseases was explored in chronic lymphocytic leukemia (CLL) and recurrent or chronic infectious mononucleosis (IM). Elevated EBV levels were demonstrated in 6/13 CLL and most IM patients, suggesting EBV involvement in CLL as well as IM. They also suggest that an increase in EBV levels in immunocompromised patients may presage increasing risk of lymphoproliferative disorders and malignancies, and signal the need for aggressive antiviral and supportive therapy. (3) A human lymphoblastoid B cell line, replicating high levels of transfer factor, was found also to produce high levels of leukocyte interferon which exhibited broad antiviral activity.